You don’t have to talk about hurricanes and tsunamis to know that the oceans are powerful. People have dreamed about harnessing their energies for centuries, and today there are many projects worldwide experimenting with just how to plug into the oceans.
However, ocean energy projects are expensive because of the nature of their energy source. The salty seas can be corrosive, unpredictable, and destructive.
Several aspects of the ocean’s energy can be exploited to generate power; we’re not limited to the crashing waves. The three most well-developed ideas are tidal power, wave power, and ocean thermal energy conversion.
There are many different projects in various stages of development in coastal states today. However, as yet, ocean energy isn’t a significant source of energy nationally.
Ocean energy is renewable, and it’s clean because of its lack of emissions. However, using ocean energy along coastlines can cause conflict with other coastal uses – transportation and scenic oceanfront – and ocean energy can as affect marine life and environmental conditions.
Wave energy capitalizes on the power of waves as they roll through the ocean. There are small wave systems generating small amounts of electricity today, though the development costs are high and it is difficult to design equipment that can withstand the salt water, weather and water pressures.
Systems have to be designed for average waves but must also withstand the much stronger waves that occur in seasonal storms and the extreme waves that appear only rarely. Waves shift direction, so systems are designed to move to optimize direction.
Prototype plants currently operating have capacities of fractions of a megawatt, which is the tiniest drop in the bucket compared to average-sized power plants in the hundreds of megawatts.
There are over 100 wave energy technologies in various states of planning and testing or in operation as prototypes. However only one type is operating commercially, the Pelamis Wave Power, according to the World Energy Council.
In the United States there are other projects in planning or testing in Hawaii, New Jersey, Oregon, Texas, and California.
Using the potential energy of rising and falling ocean tides is called tidal energy.
One way of harnessing the tides is to trap the high tide behind dams.When the ocean rises to its highest tide, the dam is closed and high water is held in a reservoir by the dam. After the water recedes in low tide, the trapped water can be released through turbines like in hydroelectric plants.
Tidal energy plants of this type demand a large height difference between high and low tides, a condition that applies to only select global locations. However, research is ongoing to bypass this limitation.
The one major tidal power plant in operation is the 240 megawatt plant in La Rance, France, which has been operating since 1966, according to World Energy Council. There is also an 18 MW experimental plant in Annapolis Royal, Nova Scotia and a 0.4 megawatt plant near Murmansk, Russia.
Tidal energy can have the same drawbacks as hydroelectric power, such that dams may interfere with aquatic life.
THERMAL ENERGY CONVERSION
Thermal energy conversion harnesses the difference in temperature between the warm, surface waters of the ocean and the colder, deep water. The two temperatures of water are matched to a fluid that has a low boiling point, like ammonia. Using the heat of the warmer water in a heat exchanger, the ammonia is evaporated and, once in gas phase, it rotates a turbine. Then, the colder seawater cools the ammonia back to liquid in a second heat exchanger. The rotating turbine generates electricity.
Open-cycle thermal energy conversion is similar but uses low pressure vessels to boil the warm surface water, instead of employing a fluid like ammonia. Water will boil at lower than its boiling point if the pressure is less than atmosphere. The steam runs a turbine, and then the cold seawater cools the steam back into fluid water.
These projects are expensive and difficult to site, since they must have deep enough water to get a substantial enough difference in temperature, yet the site must also be close enough to shore to transmit electricity.
Thermal plants can change the temperature gradient of the ocean around them, having a potential affect on marine life.
- Ocean Energy Council, World Energy Council, “Holyrood approves world’s largest tidal energy project,”Andrew Bolger, Financial Times. March 17, 2011